This invention relates generally to signal processing techniques for fiber optic interferometric sensor systems. This invention relates particularly to processing signals indicative of a phase shift in an optical signal in frequency-division-multiplexed (FDM) fiber optic interferometric sensor systems. Still more particularly, this invention relates to processing signals proportional to the sine and cosine of the phase shift in an fiber optic interferometric sensor system to determine the magnitude of the phase shift.
Common to all demodulation methods for fiber optic interferometric sensor arrays is the acquisition of an in-phase term proportional to the cosine of the interferometer phase shift and a quadrature term proportional to the sine of the interferometer phase shift. If the amplitudes of the two terms are equal, the phase shift is the arc tangent of the ratio of the quadrature term over the in-phase term. In general, the peak-to-peak amplitudes of the two terms are not equal to one another. They can vary with time depending on the noisiness of the sensor and fiber optic transmission line environments. The phenomenon of signal fading is due to changing states of polarization of the light within the fiber.
Successful implementation of the arctangent operation requires that the coefficients of the sine and cosine terms be known so that normalized terms with equal coefficients can be generated. Observation of the phase shift spinning over many cycles of 2xcfx80 arc radians is required to obtain peak values of the quadrature and in-phase terms that in the limit of many samples approach the coefficients of the sine and cosine terms.
The ratio of the peak-to-peak amplitudes of the two terms slowly drifts over time whether or not the separate amplitudes vary rapidly. This ratio can be tracked and measured when the interferometer phase shift goes through a number of cycles. Once the ratio is used to normalize or set the in-phase and quadrature peak-to-peak ratios equal to one another, the arc tangent routine is used to obtain the interferometer phase shift.
There are severe drawbacks to peak detection of the quadrature term, Q and the in-phase term, I. Observation of many samples is required to make the method work with reasonable accuracy. A sensor in a relatively quiet environment has to be observed for an excessively long time to gather a sufficient number of samples. In the limit in which peak-to-peak phase excursions are less than xcfx80 radians over the measurement time, the method fails completely.
Calibration techniques inducing shifts in the optical frequency of the light can be implemented to overcome these difficulties and force the phase shift to spin through a number of cycles of 2xcfx80 radians. However, data is not acquired during a relatively long time that might last more than one second.
The present invention provides a method for normalizing the quadrature signals output by an FDM interferometric sensor array by adding an optical phase transient on the optical signal input to the sensor array in addition to the CW phase generated carrier used for the acquisition of the interferometer phase shift. The duration of the optical phase transient is of the order of one hundred microseconds and its amplitude is of the order of 2xcfx80 radians.
Detection of an error signal associated with the transient is used to adjust the modulation depth of the CW phase generated carrier to one fixed value. A fixed value of the modulation depth produces one known ratio between the coefficients of the sine and cosine terms to allow for normalization.
The method according to the present invention for processing signals output from a fiber optic interferometric sensor array comprises the steps of applying to the sensor array a phase-generated carrier modulated optical signal at a modulation frequency f and modulating optical signals input to the sensor to produce an optical phase transient in optical signal in the sensor. An error signal associated with the transient is detected. The sensor output at frequency 2f is detected to determine the in-phase signal I. The sensor output at frequency f is detected to determine the quadrature signal Q. A normalization factor is then determined by calculating the ratio of the sensor output detected at frequency 2f to the sensor output detected at frequency f when the optical signal input to the array is modulated at a modulation depth that nulls the error signal. When the error signal is nulled, there is a fixed ratio between the signals Q and I.
The step of detecting the error signal includes the step of low pass filtering the sensor output to eliminate components having frequency greater than or equal to the modulation frequency f.